The invention relates to a method for measuring physical characteristics or physical properties of liquids including highly viscous, doughy or paste-like mediums, using an acoustic transfer system (senderxe2x80x94test sectionxe2x80x94receiver) with at least one test section formed from a solid surface, which can be brought at least partly into contact with the medium to be measured. In addition, a device for implementing the method is described.
Conventional methods for measuring viscosity generally require the removal of a trial quantity which is tested in a separate measuring device. Particularly common are rotation viscosimeters, falling sphere viscosimeters and capillary viscosimeters which use the shear gradient in the liquid on the basis of the relative motion of at least one solid surface to the liquid. A disadvantage of these methods, however, is that they cannot be integrated into a technical process, which means that great resources are necessary for a correct measurement. Measurements of products whose properties are subject to a rapid transitory change or whose properties can be easily distorted by the removal of the trial quantity or the transportation of the trial quantity are particularly difficult and are often encumbered with errors.
A viscosity sensor which is suitable for on-line measurements is described in EP 0 527 176 B1. It consists of a cylindrical main body of piezo-electric material which is connected to an alternating current source and which is activated to torsional vibrations in the ultrasound field (20-100 kHz). The vibration properties (e.g. frequency) of the main body are changed by the contiguous liquid and converted into correspondingly altered electric signals. By evaluating these signals conclusions can be drawn regarding the viscosity. This technical solution, however, makes high demands upon the material of the main body and requires great resources in the manufacture of suitable materials and the geometrical formation of the main body.
In addition, scientific and patent literature has revealed various acoustic methods and devices whichxe2x80x94using surface wavesxe2x80x94are suitable for measuring physical and/or technical dimensions (i.e. physical characteristics or physical properties) of liquids. A common factor of these methods and devices is that they are generally limited to special materials (mainly piezo-electric material, whereby sender, test section and receiver form a centralized physical entity) for the substrate of the test section and/or they rely upon defined geometric conditions (thin plates) for the sensor surface. This gives rise to disadvantages concerning the adaptability of the suggested solutions to given technical conditions (e.g. temperature, corrosivity of the medium to be measured, constructive parameters, material of the transfer section, among other things).
J. Kondoh, K. Saito, S. Shiokawa, H. Suzuki; Multichannel Shear-Horizontal Surface Acoustic wave Microsensor for Liquid Characterization; 1995 IEEE Ultrasonics Symposium, pp 445-449 disclosed the use of shear surface waves (SH-SAWxe2x86x92SHEAR HORIZONTAL SURFACE ACOUSTIC WAVE) for determining substance properties in liquids. In this case, it is a question of a special type of surface waves characterized by particle deflections solely parallel to the wave-guiding solid surface and perpendicular to the propagation direction. i.e. there are no deflection components of the particles arising perpendicularly to the surface. Even this type of wave requires the use of a piezo-electric material (e.g. LiTaO3), whereby the wave-guiding solid surface must be formed from a special crystal section.
R. M. White; Silicon Based Ultrasonic Microsensors and Micropumps; Integrated Ferroelectrics, 1995, vol. 7, pp. 353-358 describes a method for measuring viscosity using plate vibrations of a thin membrane of only a few micrometers on a silicon base. Membrane structures of this nature are, however, very sensitive particularly in relation to mechanical loads. Plate vibrations are characterized on the one hand by particle movements parallel to the surface in the propagation direction and on the other hand by particle movements perpendicular to the surface. For viscosity measurement, however, only the first-mentioned particle movements can be used.
JP 09145692 A discloses a water drop sensor for windscreens and side mirrors of vehicles or the like. The water-drop sensor consists essentially of a component (sender) generating surface waves and a component (receiver) receiving surface waves which is positioned at a distance therefrom on the surface of the body to be monitored. As soon as drops of water reach the propagation field of the surface waves between the sender and the receiver, there is a scattering of the surface waves and absorption of part of the wave energy. By means of an electronic switching circuit, an evaluation of the attenuation of the signal can be carried out. It is supposed to be possible to thereby control a windscreen wiper depending upon the quantity of water on a windscreen.
The described drop sensor can essentially only be used as a switch which triggers/does not trigger specified reactions on the basis of recognising the presence or absence of water. Due to the property of surface waves to uncouple into contiguous liquids, the measurement resultxe2x80x94thus the degree of attenuation of the signalxe2x80x94depends to a large extent upon the distribution of the water on the sensed surface. Therefore, with a dormant surface, whereby upon leaving propagation paths for the surface waves the drops form local water collections, one expects a lower attenuation than with an agitated surface with the same quantity of water, whereby the drops are distributed to a more or less even layer. Should the dynamic forces working on the water drops be so great that on the surface only a liquid layer density can form which is smaller than a quarter of the wavelength of the compression wave in the liquid, then the desired attenuation effect would not come into play.
As could be shown, the measurement signal of the drop sensor is to be understood as a total value which does not allow a differentiation between the various interactions involved in its formation. Quantitative conclusions concerning concrete physical or technical dimensions, for example the viscosity of liquids, are impossible. On the other hand, with a comparatively strong, high-volume striking of the test section with water it is expected that the wave energy used is permanently almost completely uncoupled in such a way that it is not possible to obtain a measurement signal which can be analyzed quantitatively.
EP O 542 469 A1 discloses a sensor for determining the viscosity of a liquid. The sensor has a substrate forming a sensor surface (test section) which can be brought into contact with the liquid to be investigated. Acoustic energy is made available in the form of surface waves (STW) on the sensor surface. The wave components propagate predominantly within the surface itself and only have a negligibly small vertical component propagating into the liquid. Through the use of such waves, in particular horizontal shear waves, a situation is supposedly avoided where the surface waves are too strongly attenuated during their interaction with the liquid to produce a signal which can be evaluated.
U.S. Pat. No. 4,691,714 discloses a probe device for simultaneous determination of the viscosity and temperature of a liquid. This device has a plate in which volume waves propagate, which for the purpose of determining the viscosity of the liquid interact with the liquid on a first surface which can be brought into contact with the liquid, and on whose second surfacexe2x80x94turned away from the first surfacexe2x80x94acoustic surface waves propagate, whereby the propagation speed depends upon the temperature of the probe. As the temperature of the probe is in turn influenced by the temperature of the liquid to be investigated, the temperature of the liquid to be investigated can thereby be determined indirectly.
It is an object of the invention to develop a method and a device for measuring physical characteristics or physical properties of liquids including highly viscous, doughy or paste-like mediums, which, using an acoustic transfer system with at least one test section formed from a solid surface, can be easily adapted to the most varied conditions of tangible concrete technical applications. This is achieved in particular through a multitude of substrate materials which can be used for the test section, an essentially free formation of the geometrical parameters of the transfer system and the possibility of various activation mechanisms for generating acoustic waves.
Accordingly, at least a proportion of the acoustic energy transferred in the test section is made available in the form of Rayleigh waves, whereby the Rayleigh wave propagates on a test section made from non-piezo-electric material over at least l/8, but preferably over 2l, wherein l is the wavelength of the Rayleigh wave. At least a proportion (sufficient for the measurement) of the acoustic wave energy remaining after the passage of the Rayleigh wave through one or several test sections in the measurement device is guided to and received by the receiver according to the principle of a wave guide, whereby the wave-guiding property begins at least at the beginning of the first test section (if the device has several test sections) and remains up until the receiver. i.e. the test section and the transfer path between the test section and the receiver must have a wave-guiding property. This includes both the transmission of the use signal gained in the test section with the fewest possible losses and the protection from unwanted disruptive influences. Influences on the use signal through the wave-guiding properties of the acoustic transfer system should preferably be known and ideally be optimized.
According to the invention, the wave-guiding property is provided at least between the beginning of the test section and the receiver. Otherwise, through the uncoupling of energy through the vertical component into the contiguous liquid, the information about the dimensions of the liquid to be measured which is obtained through the interaction of the Rayleigh wave with the liquid is also lost, and can therefore no longer be passed for evaluation. In order to measure the dimension of interest only, changes of at least one parameter of the Rayleigh wave are used, whereby preferably the dissipative energy loss of the Rayleigh wave in the test section is used as a basis. Essential embodiments of acoustic transfer systems with wave-guiding properties are examined in greater detail below.
The device developed to implement the method has at least one test section which can be brought at least partly into contact with the medium to be measured. This test section is suitable for forming and passing on Rayleigh waves at least a distance which corresponds to l/8 of the Rayleigh wave generated. Moreover, the acoustic transfer system is formed at least between the beginning of the test section and the receiver of the wave energy according to the principle of a wave-guide.
Through the exclusive use of Rayleigh waves for the measurement effect, the wave-guiding is largely independent of the form stability of the carrier material. The largest part of the solid is able to pass on Rayleigh waves on its surface and thus to serve fundamentally as a test section/as an acoustic transfer medium between the sender and the receiver. As the largest part of the wave energy is transported in the boundary layer (penetration depth around l) between the solid surface and the contiguous liquid, and as the Rayleigh wave has a shear component of the movement of the surface, a test section at that boundary is particularly suitable for viscosity measurement with a favorable signalxe2x80x94noise ratio.
In the strict physical sense, Rayleigh waves can only occur on unlimited, even surfaces of a solid, which may however have no significance for technical applications. Rayleigh waves nonetheless occur under these limited conditions in such a way that they can definitely be used for technical measurement purposes. Rayleigh waves within the scope of the invention are also intended to include xe2x80x9cdistortedxe2x80x9d Rayleigh waves which deviate from the xe2x80x9cideallyxe2x80x9d formed Rayleigh wave on the basis of the restrictions of the wave surface through geometric structures or boundary surfaces of materials of differing acoustic properties, or on the basis of the proximity to the sender, in whose immediate vicinity there is a not yet fully formed Rayleigh wave, or the like.
In addition the period duration of the Rayleigh wave can be determined by the construction of the transfer system and the choice of a suitable material for the test section such that the period duration is greater than the relaxation time of the liquid to be measured. For the purpose of viscosity measurement, the period duration should lie as close as possible to the relaxation time, which has a positive effect upon measurement accuracy.
When choosing materials for the purpose of adaptation to the particular application, one should consider the manufacture of alloys which are optimized in their composition or the change in elastic properties of the layer provided for passing on the Rayleigh wave, where the change is achieved through radiation with a beam which produces structural changes. Preferrably, this radiation is nuclear radiation or particle radiation (e.g. neutrons), laser treatment, ion implantation or gas implantation (e.g. hydrogenation). With such precise adaptation of the wave-guiding bodies, the smallest changes in the medium to be measured, e.g. its composition, can lead to loss of equilibrium of the coupling/uncoupling behavior of the wave energy and can be used to draw corresponding conclusions.
It should be pointed out here that by using the so-called melting-spinning process, virtually any desired alloy can be produced. The basic idea of the method consists in quickly freezing the condition of a melt through shock cooling. This generally occurs by allowing a thin beam of liquid melt to run on a rotating, cooled drum, in such a way that with cooling speeds of around 1 million Kelvin per second, amorphous bands up to a density of around 0.1 millimeter are formed. However, the materials thus produced can only be used below their re-crystallization temperature.
In principle, the invention can be used to determine all those physical and/or technical dimensions of a liquid medium which influence the propagation properties of Rayleigh waves. The parameters which can be evaluated are thus the frequency, the phase speed and the amplitude of the Rayleigh waves as well as the physical characteristics or physical properties of the medium which are directly related thereto, like e.g. the viscosity or the density. Phase transitions such as those arising during xe2x80x9cBetauungxe2x80x9d, freezing, boiling processes, cavitation or crystallisation can, however, also be detected. Further application possibilities are seen in the evaluation of non-homogenous liquids or as a humidification sensor or a cavitation sensor. For example, information concerning the condition of inner structures of non-homogenous liquids can be obtained. Preferably use is thereby made of the property of the Rayleigh wave to possess both shear components and vertical components of the movement in the boundary layer.
Although it is impossible to cite all possible application fields of the invention exhaustively, it should be pointed out that the invention can also be used in systems for process control and monitoring. This also applies for the monitoring of the condition and of the maturation process of electrochemical aggregates, e.g. fuel cells, electrolysers and batteries. In chemistry and biotechnology, removal devices for substance trials could immediately provide information concerning the substance properties.
The multitude of the substrate materials that can be used not only gives rise to good adaptability to given thermal, chemical, electrical, optical and/or mechanical conditions. The good propagation conditions of Rayleigh waves allow in many cases the use of the boundary surfaces of an existing device, e.g. of a working device or a solid wall, as a carrier for the Rayleigh wave. In this connection with carrying, it is advantageous if senders and receivers can as required be removed from the test section/from the body carrying the test section or be re-connected thereto.
A further advantage of the invention is the straightforward facility for physical separation of the sender and receiver on the one hand and the test section (measurement location) of the system on the other hand, so that for example the sender and receiver do not have to be brought into the corrosive field of the medium to be measured. Between the sender/receiver and the measurement location, a comparatively large distance can be bridged. Even the good transferability of the Rayleigh waves from one carrier to another carrier through the intermediary of an incompressible coupling medium (e.g. a liquid) can be advantageously used for bridging distances.
For the purpose of producing and verifying Rayleigh waves, greatly differing mechanisms can be used, which further increases the adaptability of the invention. The following effects can be used for production and reception of Rayleigh waves:
Mechanical/acoustic activation
Mode conversion
Piezo-electric effect
Magnetic, in particular magnetostrictive effect (As the above-mentioned effects are reversible processes, these can be used for sending and receiving.)
Thermal activation through pulsed heating, e.g. with a laser (Effect can only be used for activating a wave.)
Optical effects, including magneto-optical and electro-optical effects.
Piezo-resistive effect (The two last-mentioned effects can only be used to detect a wave.)
Means of mode conversion can be used both between sender and measurement location (test section) and between measurement location and receiver. This is normally an advantage if between the measurement location and the sender/receiver, an acoustic transfer section must be provided which cannot be arranged to pass on Rayleigh waves or can only be arranged for this purpose with difficulty. For example, a compression wave emitted from a sender at the beginning of the test section is converted into a Rayleigh wave through means of conversion (these are suitable changes in the surface structure, e.g. indentations). This method can be used particularly advantageously for guiding wave energy through solid walls (pipes, container walls).
In order to ensure the wave-guiding property of the device according to the invention, there are at least three principal variants of the invention and the sub-variants thereof. They ensure that the uncoupling of acoustic energy out of the Rayleigh wave into the contiguous liquid is suppressed and that the uncoupled energy is so far as possible completely xe2x80x9ccapturedxe2x80x9d again and fed to the receiver:
Thin liquid layer (d lkw/4)
neighboring medium with varying acoustic impedance
neighboring medium with the same acoustic impedance V(1)RW less than V(2)RW 
If the medium (medium (2)) neighboring the liquid layer which lies opposite the test section (medium (1)) has a very poor coupling ability (like e.g. vacuum, gases or foams), which results in a low degree of transmission and a high degree of reflection to this second boundary surface, the uncoupling of energy in the form of compression waves is avoided if the thickness d of the liquid layer is thinner than the liquid layer which is necessary to form the fundamental vibration of a stationary wave. With a layer thickness below lKW/4 of the compression wave this condition is normally fulfilled.
If the layer thickness of the liquid is limited by a neighboring solid (contiguous medium (2)), the condition of total reflection for the (liquid) compression wave must also be fulfilled and that opposite material neighboring the liquid layer (contiguous medium(2)) must have a higher speed VRW(2) for the Rayleigh wave than the material of the test section (contiguous medium (1)) (like e.g. glass in relation to steel, aluminum).
Opposite-lying solid boundary surfaces with acoustic coupling, preferably when d greater than lKW/4 is supposed to apply
V(1)RW=V(2)RW when reciprocal activation of Rayleigh waves on parallel solid boundary surfaces is provided
V(1)RWxe2x89xa0V(2)RW when the opposite-lying solids do not run in parallel, but their gradient is adapted to the differing Rayleigh wave speeds
V(1)RW greater than V(2)RW when the opposite-lying solidis supposed to pass on a volume sound (acoustic) wave
The uncoupling of the energy of the Rayleigh wave from the first solid boundary surface(1), which is connected to the sender, into a liquid layer with a thickness d that may be greater than lKW/4 of the compression wave, is used to couple the wave energy back to Rayleigh waves on a parallel opposite-lying second solid boundary surface(2), which forms a wave stretch with the same Rayleigh wave speed. For this purpose the sound speeds V(1)RW and V(2)RW of the first solid boundary surface(1) and the parallel opposite-lying second solid boundary surface(2) must be of equal size. From the Rayleigh wave induced on the second solid boundary surface(2) a compression wave is again uncoupled, which for its part on the opposite-lying first solid boundary surface(1) produces a Rayleigh wave again. Dependent upon the dimensions of the test section the process of the reciprocal wave activation can be formed with varying intensity. For the purpose of measuring the wave energy, the first and/or the second solid boundary surface can be provided with a receiver. The same effect can be achieved with differing Rayleigh wave speeds with adapted gradient of the boundary surfaces.
If, however, the sound speed V(1)RW of the first solid boundary surface(1) is greater than the sound speed V(2)RW of the parallel opposite-lying solid boundary surface(2), a volume sound wave is coupled into the opposite-lying solid, whereby this volume sound wave is passed into a receiver connected to this solid.
It is, however, also possible to detect directly by a receiver the compression wave uncoupled from the Rayleigh wave of the first solid boundary surface at a specified angle, and to use it for evaluating the liquid. The angle is determined by the speed VRW(1) of the Rayleigh wave in the test section and the sound speed VKW of the compression wave in the liquid.
Substrate material with slow Rayleigh wave speed (V(1)RW less than VKW)
If the substrate material of the test section(1) has a sound speed V(1)RW for the Rayleigh wave which lies below the sound speed VKW of the contiguous liquidfl, no uncoupling of energy can take place which does not serve the technical measurement purpose. In relation to most liquids (water, many oils) possible substrate materials are for example synthetic materials, soft metals (gold, lead, bismuth) as well as graphite.
It is also conceivable to use elements with acoustic-optical properties, in order to favorably influence the wave-guiding property of the acoustic transfer system.
If not only information about the liquid to be measured is required, but it is also desired to influence the properties of the liquidxe2x80x94e.g. for the purpose of controlling complex process-technical installations or for the purpose of directly influencing substance-changing processesxe2x80x94it can be advantageous if in addition to the acoustic energy, energy of another kind is coupled into the contiguous liquid layer via the solid boundary surface of the test section. This occurs via a layer which forms the boundary surface of the test section, or via the solid carrying the test section. The layer/the solid can for example be electrically conductive and be connected to a direct or alternating current source. If the liquid neighboring the test section is a polar or an electro-rheological liquid and/or a liquid with constituent parts dissociated in ions, the typical respective interactions occur in an electrical stress field. i.e. the molecules of polar liquids position themselves correspondingly in the stress field; ions migrate to electrodes with an opposite load and are discharged there if there is a sufficiently high voltage; electro-rheological liquids lose their viscosity.
It is, however, also possible to connect the layer forming the boundary surface of the test section or the solid carrying the test section to a heat source, in order to heat the contiguous liquid layer. If this layer or a layer positioned in the vicinity of the boundary surface is electrically conductive, a connection to an electrical voltage source will allow the layer to be influenced according to the principle of resistive heating.
If constituent parts of the liquid display photo-optical reactions, exertion of influence through coupling of light quanta is appropriate. For this purpose, it is recommended that the layer forming the boundary surface of the test section or the solid carrying the test section should be made to be optically permeable and that it should be connected to an energy source producing light quanta, e.g. a laser source.
According to a further embodiment of the invention, at least a part of the acoustic wave leaving the sender traverses a mode converter at least once on its way to the receiver, in order to convert its mode from a Rayleigh wave into a volume sound wave or vice versa. Through the use of mode converters it is possible to activate Rayleigh waves for measurement purposes on surfaces which are not accessible in themselves, whereby acoustic energy is passed for example through solid components, walls or similar and is converted to Rayleigh waves on the mode converter. The Rayleigh waves encumbered with the measurement information can in turn be converted into another wave mode and be forwarded to a receiver positioned at a distance from the measurement location.
The invention allows the measurement device to be formed in such a way as to ensure that senders and receivers can always be positioned outside the area in which the liquid to be measured is situated. Through the geographical separation of the test section on the one hand and the sender and receiver on the other hand, the best conditions arise for adapting the measurement device to the respective application. Furthermore, the pressures of the sender and receiver through thermal, chemical, mechanical or other influences are thereby considerably reduced/completely avoided. In many cases it will be possible to use the boundary surfaces formed from receptacle walls as a test section for acoustic waves (volume sound waves).
Accordingly, a mode converter is in an operational connection with the test section or with a field which is connected conductively to the acoustic Rayleigh waves, whereby the mode converter converts a volume sound wave running to the test section into a Rayleigh wave and/or a Rayleigh wave running back to the receiver into a volume sound wave. Mode converters are constituted by periodically positioned geometric structures whose division of wavelength I corresponds to the Rayleigh wave to be generated. Such geometric structures can for example be series of holes or wedge-like formations.
It is, however, also possible to position separate, i.e. additional elements on the test section or on the field which has a wave-guiding connection with the test section, in order to bring about a mode conversion of a volume sound wave into a Rayleigh wave or vice versa. For example, structures can be used which are stuck on, printed on, sintered on or dampened on the test section periodically with a division of around l of the wavelength of the Rayleigh wave.
Through structural measures on the mode converter, the preferred propagation direction of the Rayleigh wave, which originates in the contact of a volume sound wave with the mode converter, can be influenced. If required, a guided or symmetrical mode conversion can be achieved. If a Rayleigh wave reaches the sphere of influence of one of the above-described mode converters, this Rayleigh wave is, among other things, converted into a volume sound wave. Due to the attenuation properties of the vibration-guiding body in question and due to the conversion losses, as few mode converters as possible should lie within a path of the acoustic waves between sender and receiver.
In the event that a one-component sender-receiver unit should be provided for the purpose of generating and receiving volume sound waves by means of a simple oscillating crystal or shear vibrator, the use of only one mode converter is advantageous. It converts the volume sound wave running to the test section into a Rayleigh wave and the Rayleigh wave running back from a reflector into a volume sound wave which can be detected by the receiver. Suitable reflectors are in particular slit-like indentations with almost perpendicular flanks in relation to the propagation direction of the Rayleigh wave, but other points of discontinuity with sufficiently good acoustic reflection behavior are also suitable.
However, should the sender and the receiver (for the purpose of generating/receiving volume sound waves) be positioned far apart, the use of two mode converters which flank the test section at its ends will normally be necessary.
It is also possible to combine senders and receivers for different modes of the acoustic waves, and thus to use a sender for volume sound waves and a receiver for Rayleigh waves (or vice versa). Accordingly, a mode converter would have to be provided between the test section and the receiver/between the sender and the test section.
For the purpose of determining the physical characteristic or physical property to be measured, exclusively changes of at least one parameter of the Rayleigh wave are used, whereby a basis is preferably the dissipative energy loss of the Rayleigh wave in the test section.
The device provided for implementing the method uses an acoustic transfer system with at least one test section formed from a solid surface, whereby this test section can be brought at least partly into contact with the medium to be measured. The test section is suitable for forwarding Rayleigh waves (RW) at least on a length which corresponds to l/8, preferably more than 2l of the generated Rayleigh wave (RW). According to the invention, at least one mode converter is in an operational connection with the test section or with a field connected to the test section, which
a) converts a volume sound wave running from the sender to the test section into a Rayleigh wave and/or
b) converts a Rayleigh wave running back from the test section to the receiver into a volume sound wave.
It is also worth noting that between the sender/receiver and the measurement location a comparatively large distance can be bridged, as Rayleigh waves can with a comparatively low energy loss cover long stretches due to their low angle straggling. The good transferability of the Rayleigh waves from one carrier to another carrier by means of an incompressible coupling medium (e.g. a liquid) can also be advantageously used for bridging distances.